In such devices, the plate and cone (or plate and plate), are arranged one above the other and form the two measuring surfaces. Between them is located the substance whose viscosity is to be measured. When the two surfaces are counter-rotated relative to each other, one or both the surfaces being driven, at least one of these surfaces cleaves or slices through the substance to be measured.
A slicing-through action in visco-elastic substances causes not only tangential but also normal forces to occur which tend to force the two surfaces apart. To determine these normal forces, typically the force acting on the lower measuring surface is measured. This surface is either the plate or the cone and may be driven or stationary. High-polymer substances and their solutions belong to the group of visco-elastic substances. One example is the polymers used to produce fibres. After they emerge through the nozzles of an extruder the substance expands perpendicular to its elongated direction, i.e. its direction of stress. The forces which cause this expansion are determined in the rotary viscometer by the device for measuring these normal forces.